Bragg cells have been used extensively as acousto-optic modulators in optical processing systems that operate on wide bandwidth (e.g. R.F.) signals. As an acousto-optic device, the Bragg cell operates to convert an electrical (wideband RF) signal into an optical signal and serves as a basic storage element of the optical signal processor. Among the various optical processing systems that have employed Bragg cells in carrying out computationally intensive operations are spectrum analyzers, correlators and ambiguity function generators.
Within the optical signal processor, the Bragg cell is typically used as a single channel device, in which a wideband signal is applied to a transducer mounted on one end of the bulk medium of the cell and an acoustic wave is launched into the bulk from the transducer As the acoustic wave propagates into the cell, it spatially modulates an impinging optical beam, which is then subjected to downstream optical processing. In addition to such single channel cell configurations, Bragg cells may also be driven as high performance multi-channel devices, such as very high bit rate optical recorders. Here, instead of a single transducer the Bragg cell has a plurality of transducers disposed adjacent to one another on one face of the bulk, to which the multiple bits of the information signal to be recorded are applied. In this application, only a single resolution element is typically stored within the bulk along the length of the cell (i.e. in the direction of propagation of the acoustic wave), so that the cell effectively operates in a one-dimensional (down the cell) linear array format.
In addition to these one-dimensional processing schemes, it would be desirable to process signals in a two-dimensional format. To use a Bragg cell for two-dimensional processing, it is necessary to store a significant portion of the time history of each signal to be processed along the length of the cell. Unfortunately, because of physical constraints on the size of system components, multi-channel Bragg cells typically employ transducers that are small and closely spaced, in order to provide a large number of channels on a small crystal. As a result, when the acoustic waves that are launched by the respective transducers of the cell propagate into the bulk from the face of the crystal on which the transducers are mounted, the waves expand or spread, similar to the diffraction pattern produced by an aperture antenna.
This spreading phenomenon is illustrated in FIG. 1, which shows a portion of a multi-channel Bragg cell 10 of length L on one face 11 of which are mounted a plurality of transducers 12, 13 and 14 associated with respective channels of the cell In response to the application of electrical signals, the transducers 12, 13, 14 launch into the Bragg cell 10 respective acoustic waves 22, 23, 24 which spread out as they propagate across the cell 10 and eventually overlap one another before reaching the other side of the cell. In the illustration of FIG. 1, acoustic wave 22 is shown as overlapping acoustic wave 23 in region 32, and acoustic wave 23 is shown as overlapping acoustic wave 24 at region 33. These regions of overlap 32 and 33 create interference in the signals to be processed (as the information is no longer in separate, discrete channels), so as to effectively reduce the useful length of the Bragg cell from its maximum available propagation length L to some lesser dimension L'. This reduction in the effective length of the Bragg cell consequently decreases the time-bandwidth product of each channel.
Reports of studies of this diffraction spreading phenomenon have indicated that, although the beams overlap, information in the phase front of each beam can be used to direct the beams into distinct channels. More particularly, as described in articles by A Vanderlugt et al, entitled "Multichannel Bragg Cells: compensation for acoustic spreading", Applied Optics, Vol. 22, No. 23, Dec. 1, 1983, pp 3906-3912 and "Acoustic spreading in multichannel Bragg cells" SPIE Vol 465, Spatial Light Modulators and Applications (1984) pp 152-155, and in a previous article by I.A. Vodovatov et al, Pis'ma Zh. Tekh Fiz 7, 369 (1981), [Sov. Tech. Phys. Lett. 7, 159 (1981)], p. 159 which is referenced in the Vanderlugt et al articles, maintaining distinctness among the multiple channels can be accomplished by using a holographic optical element created from one of the channels in the Bragg cell that is driven by a pure frequency. A reference beam, equally offset in frequency, may be used to capture the amplitude and phase information in a one-dimensional hologram. When this element is placed in the Fourier transform plane of a lens disposed downstream of the Bragg cell, the amplitude and phase for each channel is matched so that, in an image plane of the Bragg cell, each beam is distinct An illustration of the compensated cell is shown in FIG. 2. With this compensation, the signal applied to each transducer 16, 15, 14 of the cell 10 propagates as a nondiverging beam 22', 23', 24'.
Although the above described proposal provides compensation for acoustic spreading, because of the manner in which the optical energy is compressed, there may result a loss of the modulation transfer function which increases as a function of the distance from the transducer and therefore may affect subsequent optical processing.